2.1. ALZHEIMER'S DISEASE
Alzheimer's disease, which is the single major cause of dementia in adults in industrialized societies, is a degenerative brain disorder characterized clinically by a progressive loss of memory, confusion, dementia and ultimately death. Histopathologically, Alzheimer's disease is characterized by the presence in the neocortex, especially the hippocampus, of two brain lesions: the neurofibrillary tangles (NFTs) of paired helical filaments (PHF) in the neurons and the neuritic (senile) plaques in the extracellular space. The formation of senile plaques is related to the appearance of the symptoms and signs of the disease, including amnesia. After the formation of senile plaques, neurofibrillary tangles are produced in the neuronal bodies. The formation of neurofibrillary tangles is related to the worsening of amnesia and other symptoms of dementia.
There are several animal models of Alzheimer's disease. In one model, the insertion and overexpression in mice of a mutant form of the human amyloid precursor protein gene as a minigene under the regulation of the platelet-derived growth factor .beta. receptor promoter element causes the appearance of a syntaptic pathology and amyloid-.beta. peptide deposition in the brain of the mice. These changes in the brains of transgenic animals are similar to those seen in human Alzheimer's disease (Games et al., 1995, Nature 373:523-527).
In a similar model, transgenic mice were generated by using a promoter derived from the mouse neurofilament-light gene to drive expression in nerve cells of the mouse 42-amino acid A.beta. protein (LaFerla et al., 1995, Nat. Genet. 9:41-47). Such mice also exhibit insoluble A.beta. deposition in the brain.
A major component of the amyloid deposits in Alzheimer's disease is a polypeptide referred to herein as A.beta. (Amyloid-beta). A.beta. also accumulates in the wall and the lumen of the brain vessels. A.beta. is normally a soluble component of the cerebrospinal fluid where it is found in concentrations of about 3-5 nM. A.beta. may have 39 to 43 amino acids, typically 40 amino acids, in the mature form and is derived as a proteolytic cleavage product from a cell surface protein called the amyloid precursor protein (APP) (Kang et al., 1987, Nature 325:733-736). The normal function of A.beta. is not known at present but might be to form cation selective channels across cell membranes (Kawahara et al., 1997, Biophysical Journal 73/1, 67-75).
The major form of Alzheimer's disease is sporadic and has a late onset, whereas a small percentage of cases are familial and have an early onset. Some of the familial cases of Alzheimer's disease are strongly associated with one or more mutations at different sites on the A.beta. precursor protein, the gene of which lies on chromosome 21. Whether these mutations are the cause of Alzheimer's disease in the affected patients, however, has not been proven experimentally.
The plaques are not unique to Alzheimer's disease. The senile plaques are also seen in Down syndrome and in both aged human and animal brains. The numbers of plaques in non-demented aged humans are sometimes similar to those seen in Alzheimer's disease cases (Katzman et al., 1988, Ann. Neurol. 23:138-144).
The precipitation of synthetic A.beta. has been shown to be caused by several environmental factors including low pH, high salt concentrations and the presence of metals, e.g., zinc, copper, and mercury (Bush et al., 1995, Science 268:1921-1923). It has been reported that A.beta. itself specifically and saturably binds zinc with a high affinity binding (K.sub.D =107 nM) at a molar ratio of 1:1 (zinc: A.beta.) (Bush et al., 1994, J. Biol. Chem. 269:12152-12158). This binding takes place at physiological concentrations of zinc (Bush et al., 1994, Science 265:1464-1467).
There is a strong supposition that the removal of amyloid deposits from patients suffering from Alzheimer's disease will alleviate the symptoms of Alzheimer's disease. Therefore, several attempts have been made to prepare a drug for the removal of amyloid deposits, as methods for healing Alzheimer's disease are urgently sought.
International Publication No. WO 93/10459, dated May 27, 1993, discloses a method for the treatment of Alzheimer's disease by administering a zinc binding agent. As preferred compounds, phytic acid, desferri-oximine, sodium citrate, EDTA, 1,2-diethyl-3-hydroxy-pyridin-4-one, and 1-hydroxyethyl-3-hydroxy-2-methyl-pyridin-4-one are mentioned.
German publication DE 39 32 338, dated Apr. 11, 1991, discloses the use of an aluminum chelator, such as 8-hydroxy-quinoline, for the treatment of Alzheimer's disease.
U.S. Pat. No. 5,373,021, dated Dec. 13, 1994, discloses disulfiram and its salts and analogs. According to this patent, disclosed compounds may be used to reduce neurological damage caused by Alzheimer's disease.
The hitherto known compounds suggested for the treatment of Alzheimer's disease have several drawbacks, which has prevented their widespread use. Most of the compounds are unable to penetrate the blood-brain-barrier and thus cannot readily reach the areas in which the amyloid is deposited. Disulfiram, which may penetrate the blood-brain-barrier, has the drawback that when it is combined by a patient with ethyl alcohol, it causes severe adverse reactions, including headaches, nausea, vomiting, sweating, thirst, weakness, and low blood pressure.
The most extensively studied chelator is EDTA. However, the chelating effect of EDTA is relatively weak towards zinc and copper. Furthermore, EDTA cannot penetrate the blood-brain-barrier and is considered relatively toxic.